The earnings per share alone means absolutely nothing, though. To look at a company's earnings relative to its price, most investors employ the price/earnings (P/E) ratio. The P/E ratio takes the stock price and divides it by the last four quarters' worth of earnings. For instance, if, in our example above, XYZ Corp. was currently trading at $15 a share, it would have a P/E of 15.

   $15 share price
---------------------------= 15 P/E
$1.00 in trailing EPS

Is the P/E the Holy Grail?
There is a large population of individual investors who stop their entire analysis of a company after they figure out the trailing P/E ratio. With no regard to any other form of valuation, this group of unFoolish investors blindly plunge ahead armed with this one ratio, purposefully ignoring the vagaries of equity analysis. Popularized by Ben Graham (who used a number of other techniques as well as low P/E to isolate value), the P/E has been oversimplified by those who only look at this number. Such investors look for "low P/E" stocks. These are companies that have a very low price relative to their trailing earnings.

Also called a "multiple", the P/E is most often used in comparison with the current rate of growth in earnings per share. The Foolish assumption is that for a growth company, in a fairly valued situation the price/earnings ratio is about equal to the rate of EPS growth.

In our example of XYZ Corp., for instance, we find out that XYZ Corp. grew its earnings per share at a 13% over the past year, suggesting that at a P/E of 15 the company is pretty fairly valued. Fools believe that P/E only makes sense for growth companies relative to the earnings growth. If a company has lost money in the past year or has suffered a decrease in earnings per share over the past twelve months, the P/E becomes less useful than other valuation methods we will talk about later in this series. In the end, P/E has to be viewed in the context of growth and cannot be simply isolated without taking on some significant potential for error.

Are Low P/E Stocks Really a Bargain?
With the advent of computerized screening of stock databases, low P/E stocks that have been mispriced have become more and more rare. When Ben Graham formulated many of his principles for investing, one had to search manually through pages of stock tables in order to ferret out companies that had extremely low P/Es. Today, all you have to do is punch a few buttons on an online database and you have a list as long as your arm.

This screening has added efficiency to the market. When you see a low P/E stock these days, more often than not it deserves to have a low P/E because of its questionable future prospects. As intelligent investors value companies based on future prospects and not past performance, stocks with low P/Es often have dark clouds looming in the months ahead. This is not to say that you cannot still find some great low P/E stocks that for some reason the market has simple overlooked -- you still can and it happens all the time. Rather, you need to confirm the value in these companies by applying some other valuation techniques.

The PEG and YPEG
The most common Foolish applications of the P/E are the P/E and growth ratio (PEG) and the year-ahead P/E and growth ratio (YPEG). Rather than reinvent the wheel, as there is a wonderful series of articles already written on these very subjects in Fooldom, I will simply direct your attention to them and talk about them very briefly. The PEG simply takes the annualized rate of growth out to the furthest estimate and compares this with the current stock price. Since it is future growth that makes a company valuable to both an acquirer and a shareholder seeking either dividends or free cash flow to fund stock buybacks, this makes some degree of intuitive sense. Only looking at the trailing P/E is kind of like driving while looking out the rearview mirror.

If a company is expected to grow at 10% a year over the next two years and has a P/E of 10, it will have a PEG of 1.0.

P/E of 10
----------------------   =  1.0 PEG
   10% EPS growth

A PEG of 1.0 suggests that a company is fairly valued. If the company in the above example only had a P/E of five but was expected to grow at 10% a year, it would have a PEG of 0.5 -- implying that it is selling for one half (50%) of its fair value. If the company had a P/E of 20 and expected growth of 10% a year, it would have a PEG of 2.0, worth double what it should be according to the assumption that the P/E should equal the EPS rate of growth.

While the PEG is most often used for growth companies, the YPEG is best suited for valuing larger, more-established ones. The YPEG uses the same assumptions as the PEG but looks at different numbers. As most earnings estimate services provide estimated 5-year growth rates, these are simply taken as an indication of the fair multiple for a company's stock going forward. Thus, if the current P/E is 10 but analysts expect the company to grow at 20% over the next five years, the YPEG is equal to 0.5 and the stock looks cheap according to this metric. As always, one must view the PEG and YPEG in the context of other measures of value and not consider them as magic money machines.